Quiz 3: Pre/Postcondition Change

1. Which condition is weaker than \(x > 0\)?

  • \(2x > 0\)
  • \(x \cdot x > 0\)
  • \(x + y > 0\)
  • \(x > 1\)

Correct answer: \(x \cdot x > 0\)

Explanation:

Recall: \(P_1 \iff P_2\) means the conditions are equivalent.

  • \(x > 0 \iff 2x > 0\), so the conditions are equivalent.
  • \(x > 0 \implies x \cdot x > 0\), so \(x > 0\) is stronger.
  • \(x > 0\) and \(x + y > 0\) aren't comparable because \(y\) isn't included in the implicant.
  • \(x > 1 \implies x > 0\), so \(x > 1\) is stronger.

2. Which statement is correct?

  • \(\text{length} = \text{count}\) is stronger than \(\text{length} = (\text{count}-1)\)
  • \(\text{length} = \text{count}\) is neither stronger nor weaker than \(\text{length} = (\text{count}-1)\)
  • \(\text{length} = \text{count}\) is weaker than \(\text{length} = (\text{count}-1)\)
  • \(\text{length} = \text{count}\) is equivalent to \(\text{length} = (\text{count}-1)\)

Correct answer: \(\text{length} = \text{count}\) is neither stronger nor weaker than \(\text{length} = (\text{count}-1)\)

Explanation:

It doesn't make sense to consider the implication relationship \[\text{length} = \text{count} \implies \text{length} = (\text{count}-1)\] or vice-versa. Thus, there is no relationship between their contract strengths.

3. Which of the following is the most reasonable precondition of the method deposit(double amount) in the BankAccount class?

  • \(\text{amount} \neq 0\)
  • \(\text{amount} \geq 0\)
  • \(\text{amount}\) is any double value
  • \(\text{amount} > 0\)

Correct answer: \(\text{amount} > 0\)

Explanation:

The action of "depositing" at a bank only makes sense when you're depositing a positive amount of money. This rules out all but \(amount > 0\).

4. In the following code, Module B results in a call of sqrt(5). Which statement is correct?

// Module A
// Precondition: x >= 0
// Postcondition: return x's square root

double sqrt(double x) {
    ...
}

// Module B

double x = -5;
double y = sqrt(x);

assert abs(y * y-x) < eps
  • There is no bug in either A or B
  • There is a bug in both A and B
  • There is a bug in A
  • There is a bug in B

Correct answer: There is a bug in B

Explanation:

Recall the Pre/Postcondition Violation Rules.

Client B violates Supplier A's precondition, so the client (B) is at fault.